A car went off a freeway ramp in Seattle on Monday morning, killing a 19-year-old man inside a tent in a small homeless encampment, said the Washington State Patrol.

The dead man was identified by the state patrol on Monday afternoon as Walter Burton.

The driver, identified as 33-year-old Oscar Gutierrez de Jesus, of Seattle, fled the scene but was arrested soon after for investigation of death by dangerous driving.

The tent pitched in the grassy central reservation of Interstate 5 is among hundreds of unauthorised camps that have popped up on vacant land, along freeways and in neighbourhoods throughout Seattle. 

Like many cities, Seattle has been wrestling with how to deal with growing numbers of homeless people. According to 2015 data from the US Department of Housing and Urban Development, Seattle has the third highest population of homeless people in the country, behind Los Angeles and New York.

A one-night census of homeless in January revealed a 19 percent jump in Seattle, the third annual increase in as many years.

Last autumn, Seattle Mayor Ed Murray proclaimed a state of emergency and called on state and federal officials for support.

The city has also been wrestling with how to clean up unauthorised encampments, as it considers public safety concerns as well as how best to serve those living in the outside spaces.

The encampment where Monday's crash happened is on state transportation land. It is one of the more well-known areas where tents are pitched, and it was scheduled to be cleaned up in mid-October, state transportation spokeswoman Kris Olsen said.

In many cases after the clean-up, crews often find that the former residents move nearby or return a few days later, or new people move in, Olsen said.

"It's a persistent problem, but the bottom line is our top priority is safety," she said. "While we're sympathetic to people experiencing homelessness, this morning's incident demonstrated that these areas are just not a safe place for any member of the public."

Since November, the city has conducted 441 sweeps of illegal homeless encampments. During that time, only 126 people received permanent housing or shelter services while outreach workers talked to about 18,50 people.

The mayor has announced a new task force to review the city's policies on encampment sweeps.

Several Seattle City Council members have proposed legislation to make changes as well.

Source: Agencies